1. Field of the Invention
The invention relates in general to planar antenna structures. In particular the invention relates to a planar structure combining two different antenna architectures, thus operating at two clearly distinct frequencies. In addition, the invention relates to the feed arrangement of such an antenna and to a radio apparatus employing such an antenna.
2. Description of the Related Art
FIG. 1 shows a known basic design 100 of a planar inverted-F antenna (PIFA) comprising a planar electrically conductive radiating element 101, electrically conductive ground plane 102 parallel to said radiating element, and, interconnecting these two, a ground contact 103 which is substantially perpendicular to the radiating element and ground plane. The structure further includes a feed electrode 104 which also is substantially perpendicular to the radiating element and ground plane and which can be coupled to an antenna port (not shown) of a radio apparatus. In the structure of FIG. 1 the radiating element 101, ground contact 103 and the feed electrode 104 are usually manufactured by cutting a thin metal sheet into a suitable rectangular shape which has got two protrusions bent to a right angle. The ground plane 102 may be a metallized area on the surface of a printed circuit board so that the ground contact 103 and feed electrode are easily connected to holes on the printed circuit board. The electrical characteristics of the antenna 100 are affected in general by the dimensions of its elements and in particular by the size of the radiating element 101 and its distance from the ground plane 102.
A disadvantage of the antenna structure depicted in FIG. 1 is its poor mechanical stability. Various structures have been proposed to solve this problem. European Patent document EP 484,454 discloses a PIFA structure according to FIG. 2 wherein a radiating element 201, ground plane 202 and a ground contact 203 interconnecting these two are realized as metal platings on surfaces of a solid dielectric body 204. The antenna is fed through a coupling element 205 which does not touch the radiating element 201. An electromagnetic coupling exists between the coupling element 205 and radiating element 201, and the coupling element extends over the edge of the dielectric body 204 to a point that can be coupled to the antenna port of a radio apparatus. The structure is mechanically stable, but the dielectric body block makes it rather heavy. Furthermore, the dielectric body decreases the impedance bandwidth of the antenna and degrades the radiation efficiency compared with an air-insulated PIFA.
A PIFA radiating element does not have to be a simple rectangle as in FIGS. 1 and 2. FIG. 3 shows a known PIFA radiating element 301 design. The rectangular shape is broken by a slot 302 which forms a sort of strip in that portion of the radiating element which is farthest away from the feedpoint 303 and ground contact 304. The purpose of the slot usually is to increase the electrical length of the antenna and thus affect the antenna's resonant frequency.
All the PIFA structures described above are designed such that they have a certain resonant frequency as well as an operating frequency band centering round said resonant frequency. In some cases, however, it is preferable that the antenna of a radio apparatus has two different resonant frequencies. FIGS. 4a and 4b show dual-frequency PIFA radiating elements known from the publication "Dual-Frequency Planar Inverted-F Antenna" by Z. D. Liu, P. S. Hall, D. Wake, IEEE Transactions on Antennas and Propagation, Vol. 45, No. 10, October 1997, pp. 1451-1457. In FIG. 4a the antenna comprises a rectangular first radiating element 401 and a second radiating element 402 surrounding said first radiating element from two sides. The first radiating element has a feedpoint 403 and ground contact 404 of its own, and the second radiating element has those of its own, 405 and 406. In FIG. 4b the antenna comprises a continuous radiating element 410 which is divided into two branches by a slot 411. The feedpoint 412 is located near the inner end of the slot 413, i.e. the end that does not end at the edge of the radiating element, so that the branches have different directions from the feedpoint on. Both branches have electrical lengths of their own which differ from each other considerably. The ground contacts 413 are located near the edge of the structure.
It is further known a dual-frequency PIFA radiating element 501 according to FIG. 5 which has two branches in the same manner as the radiating element in FIG. 4b. In FIG. 5, the outermost ends of both branches extend to the edge of the printed circuit board, depicted in the figure by the dashed line, which supports the radiating element. This structure provides a somewhat wider antenna impedance band, i.e. frequency range around a particular resonant frequency in which the antenna impedance matching to the antenna port of the radio apparatus is good. At the same time, however, the SAR value, which indicates the amount of radiation absorbed by the user, becomes rather high, especially in the higher frequency band.
Finnish patent application FI-982366 discloses a PIFA radiating element 600 according to FIG. 6, in which said radiating element is divided by a non-conductive slot 601-602-603 which divides the planar radiating element into a first branch and second branch. The feedpoint 604 and ground contact 605 are located close to the inner end of the slot. So, this structure, too, has two adjacent PIFA radiating element branches on one and the same planar surface and in the vicinity of one and the same ground plane 606. The patent application also discloses that the outer end of the branch corresponding to the higher operating frequency is located within the border line of the radiating element, surrounded by the first branch so that the SAR value will be smaller than in the arrangement of FIG. 5.